Dark matter is arguably one of the universe’s most perplexing mysteries. Astronomers have gathered overwhelming evidence that it makes up roughly 84% of the universe's matter. Its extra gravity provides the most straightforward explanation for the rotations of individual galaxies, the motions of distant galaxy clusters, and the bending of distant starlight.

So what is this elusive matter? A popular theory is that it consists of a yet-undiscovered exotic massive particle that barely interacts with normal matter. These particles have so far eluded detection. But theoretically they act as their own antiparticles, and can annihilate to produce a cascade of familiar particles, including electrons and positrons. The collision should generate gamma-rays — the most energetic photons in nature.

NASA’s Fermi Gamma-Ray Space Telescope has been scouring the sky in search of this tell-tale annihilation signature since its launch in 2008. While the telescope has spotted a large number of gamma rays pouring outward from the center of our galaxy, astronomers have not been able to determine if this detection is due to dark matter annihilation or other natural particle accelerators.

The most likely culprits for the latter alternative are undetected pulsars. These rotating neutron stars beam huge amounts of energy out of their poles, including matter-antimatter pairs that can annihilate in bursts of gamma rays.

A team of astronomers led by Tansu Daylan (Harvard University) has further scrutinized the excess Fermi signal, and has ruled out pulsars as the cause. This leads to the conclusion that the signal must be due to annihilating dark matter — a claim that would resolve one of the biggest mysteries in physics.

“If our interpretation is correct, this signal would constitute the discovery of an entirely new particle that makes up the majority of the mass found in the universe,” says coauthor Dan Hooper (Fermi National Accelerator Laboratory). “I can't find words that are strong enough to capture the significance of such a discovery.”

The problem is the galactic center is extremely bright and dense. Billions of stars give off an incredible amount of light alone, making the core hard to map precisely and uncover hidden sources of gamma-ray radiation, such as pulsars. The team was able to precisely comb through the data and subtract known sources of gamma rays, ultimately producing a sharp map that extends nearly 5,000 light-years in all directions away from the galactic center.

There are far fewer stars so far away from the galactic center, and if pulsars were indeed the cause of the gamma-ray signal, we would be able to see several as individual gamma-ray sources. But we don't. That fact is compelling evidence that pulsars do not exist at such a large distance away from the galactic center, and that the excess gamma-rays must be due to dark matter annihilation alone.

If this is the case and the glimmer of gamma-rays from the inner galaxy is the afterglow from annihilating dark matter particles then their detected energy levels indicate that they most likely originate from dark matter particles with a mass range from 31 to 40 GeV.

But others remain skeptical. “ ‘Extraordinary claims need extraordinary evidence,’ as we said about the B-mode signal recently,” says expert Kevork Abazajian (University of California, Irvine), referring to the discovery of inflation’s fingerprint on the cosmic microwave background announced last week.

Abazajian argues that the population of pulsars located in the galactic center may be different than pulsars in the spiral arm near us. They could be dimmer, so we wouldn’t necessarily pick them up as point sources like we expect to. This idea may seem far-fetched but “it’s a basic principle of science: If you have something extremely novel you have to make sure you’ve taken into account every other possibility,” says Abazajian.

To verify the gamma-ray excess, astronomers are pointing their telescopes toward less bright sources, particularly dwarf galaxies. Such dim sources are expected to be rich in dark matter but not in other natural particle accelerators, such as pulsars. However, there is currently too little data to determine whether there is a similar excess emanating from these dwarf galaxies.

Many scientists argue the most convincing evidence for dark matter particles will come from mine shafts deep underground or massive particle accelerators, where physicists are working hard to directly detect the impacts of individual particles. While no direct detection has been made yet, a particle mass of 31 to 40 GeV could be seen with the Large Hadron Collider.

It may take another few years, but astronomers are potentially on the verge of cracking one of the universe’s most compelling mysteries.

About Shannon Hall

Shannon Hall, a freelance science journalist, has two bachelors in physics-astronomy and philosophy and two masters in astronomy and science journalism. Eight years of higher education explains why she's hooked on the smell of freshly ground coffee almost as much as the wonders of the universe.

13 thoughts on “Dark Matter Spotted in the Milky Way?”

Re your opening paragraph… "Dark matter is arguably one of the universe’s most perplexing mysteries. Astronomers have gathered overwhelming evidence that it makes up roughly 84% of the universe."

I really think you meant to say that dark matter "makes up roughly 84% of the [combination of baryonic and non-baryonic matter in the] universe [or roughly 26.8% of the total universe]." Or words to that effect. As it stands now, you’ve completely left out dark energy which, by current theory, accounts for roughly 68% of the universe. Since 84% plus 64% comes out at roughly 152% the numbers simply don’t add up since, in the end, the universe can’t be comprised of more than 100% of itself. 😉

I remain skeptical of dark matter’s existence but this observation could sway me eventually. There is so much we don’t know about gamma ray emitters in the universe, though. May I point out that it wasn’t until the 1990s that gamma rays were discovered shooting up into space from the tops of terrestrial thunderstorms. (This discovery was made from 250 miles in space by the Compton Gamma Ray Observatory.) It seems to me that pulsars (conventional) and dark matter (unconventional) may not be the only explanations for the observations. I would like to see more done to consider and eliminate other possibilities, both known and unknown, before accepting the "dark matter" explanation.

I too remain skeptical. What I would love to see is a graph plotting star size versus number of stars. Are red dwarfs (the most numerous of stars) on the etreme right hand side of a bell curve? Are brown dwarfs millions/billions/trillions of times more common?

I share Paul’s skepticism about the existence of weakly interacting massive particles, the currently favored explanation for the gravitational anomalies observed at galactic and intergalactic scales. Maybe WIMP’s exist, but until somebody finds and measures some of them, WIMP’s could just as well be our version of aether, a physical substance supposed to exist because without it our theoretical understanding of the universe is unable to explain observed phenomena. I don’t know enough about the details of modified Newtonian dynamics to judge whether MOND is a likely explanation for the strange ways galaxies rotate and cluster, but I find compelling the basic approach to look first for shortcomings in our theories rather than magical particles that are conveniently invisible. My layperson’s hunch is that the search for WIMP’s will turn out to be a massive snark hunt.

Is Dark Matter affected by gravity such that one would expect to find more of it in a galaxy rather than in intergalactic space?
Can we detect, or have we detected objects in interstellar and intergalactic space that occulated stars or "large flat field light sources" like nebulas and galaxies so that we can see a pattern of blinking as the near object passes in front of many far light sources?

Anthony’s comment sums up my skepticism about WIMPs better than I did. I like his analogy to "aether". May I add the analogy of the Ptolemaic system? Then, as now, observations (planetary motions ) were made that were discordant with contemporary (geocentric) theory. Ptolomey introduced an increasingly complex set of epicycles and deferents to bring these observations in line with the existing paradigm. So, it seems to me, are the increasingly complicated, ever-changing theoretical properties of "dark matter" to explain observations of certain (but not all) phenomena that are very discordant with our current understanding of gravity. While I understand that science cannot be done by analogy, it is a useful tool for keeping current paradigms in perspective.

"While no direct detection has been made yet, a particle mass of 31 to 40 GeV could be seen with the Large Hadron Collider." So why hasn’t it? The Higgs boson discovery came in at about 125 GeV; why hasn’t a dark matter "resonance" been seen at the LHC or previous experiments? Why is the Higgs boson so much easier to produce than dark matter?!? Also, they did not find a signal, then try to explain it; they looked for a signal…and found it! It is a well-known psychological effect that if you look for needles in haystacks long and hard enough, eventually you will start to see needles. Yet, reading the paper is pretty convincing. Their graphs seem to show a real signal. The conclusion just doesn’t feel right. Now I know how Einstein felt about quantum mechanics…

Nice article Ms Hall, and interesting comments gentlemen. Doubts of dark matter’s very existence seem misplaced, because bulk intra- and extra- galactic motions require its presence. And there is a real way to detect its existence in places: gravitational lensing of background objects like stars and galaxies. Skepticism about WHAT the missing, dark 84% of matter may in fact be IS warranted however until the “extraordinary evidence” comes in. Shannon reports that the theorized weakly interactive massive particles (wimps) may “act as there own antiparticles” which is a property of bosons. They would have to be stable, unlike the Higgs boson. My probably simplistic questions are: Why wouldn’t these supposed particles be attracted to and fall into, thereby contributing to, the masses of all very massive bodies, since a conjecture of this paper seems to be that the gravity wells in galactic cores make it more likely that these particles will hit each other in a core’s vicinity? Why wouldn’t wimps occasionally collide and annihilate in the cores of stars, giant planets, etc., etc.? And why is so much of this dark stuff perpetually suspended in the outer halos of spiral galaxies, thereby enabling them to maintain their shapes over immense spans of time? Why wouldn’t this halo dark matter also tend to settle into the denser and therefore gravitationally attractive disk areas of spirals just like ordinary matter does, if they are just free particles? Isn’t it easy to see why the jury has reasonable doubts?

One the one hand, it is assumed that WIMPs occasionally collide and annihilate in the cores of stars, giant planets, etc, but this reaction is as inaccessible as the trillions of tons of gold and platinum at the bottom of Earth’s iron core. On the other hand, dark matter, by definition, does not radiate, so it clumps very inefficiently, which is why it is appears mostly in galactic halos, and is so hard to detect. Gas cannot clump to form stars or galaxies except by radiating away gravitational and thermal energy. A super nova may compress gas around it, but the gas will just spring back to its former diffuse state if it does not radiate away energy while it’s hot. One question about the paper: they assume dark matter density decays exponentially away from the galactic center, as if it has reached perfect thermal equilibrium with the galaxy‘s gravitational well. But this puts the bulk of the galaxy’s dark matter right in the middle. Isn’t dark matter characteristically lurking in the galactic halo, like it should? Am I reading their paper correctly? Does their model not put most of the galaxy’s dark matter in its center, not its halo?

OK, here are my skeptic ideas. I’d like to see these addressed in a future article.
–
What is the amount of this gamma ray flux? We could assume that if this flux were high enough then the 84% of matter being dark is the end result of large self annihilations, and so the percent of matter being dark must have been higher in the past, perhaps 90, 95 or even a higher percent in the distant past. True? If true what are the implications? Could a very large value for dark matter be implicated in the formation of supermassive black holes?
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So, just to be clear, what does this flux tell us about the density of dark matter, and in particular is the density too low to observe this flux directly, unlike the particle interactions in the Lamb shift?

You say it’s massive enough to affect gravity, but doesn’t much react otherwise.
Can it be detected on the very small scale? Sort of like a ballance beam scale with nothing on one side & something that has Dark Matter on the other? Problem is getting "nothing" on one side. Not only would there be nothing there; there wouldn’t even be a means of graphing empty space.
Does it clump up like mass does due to gravity or is it everywhere equally? I would guess the orbits of stars around a galaxy probably has that answer.
Could it be some interaction with some other dimension that is more or less uniform or clumps up so that the same sort of things are in both? By that I mean, in another universe is there another earth?
Imagine this thought; there are "ghosts" that we don’t see walking right beside us but on another Earth?
If Dark Matter is being anialated, is it decreasing in amount, or constant due to some means of generation?

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